4,4′-Dichlorodiphenyl sulfone is used especially as a monomer in the synthesis of polyarylene ether sulfones. Examples of commercial significance are polyether sulfone (polymerization of 4,4′-dihydroxydiphenyl sulfone with 4,4′-dichlorodiphenyl sulfone), polysulfone (polymerization of bisphenol A with 4,4′-dichlorodiphenyl sulfone) and polyphenylene sulfone (polymerization of 4,4′-dihydroxybiphenyl with 4,4′-dichlorodiphenyl sulfone). 4,4′-Dichlorodiphenyl sulfone is thus a central unit for the preparation of these industrial polymers.
A preferred reactant fo the preparation of polyarylene ether sulfones is high-purity 4,4′-dichlorodiphenyl sulfone, since exclusively the 4,4′ isomer forms linear, nonangular polymers which have the desired product properties, for example chemical and thermal stability, high dimensional stability and flame retardancy.
Processes for preparing 4,4′-dichlorodiphenyl sulfone are known from the prior art. The known processes comprise especially the preparation proceeding from monochlorobenzene and a sulfonating agent via 4-chlorobenzenesulfonic acid as an intermediate, which is generally not isolated.
DE 2252571 describes the synthesis of dichlorodiphenyl sulfone from monochlorobenzene and sulfuric acid at temperatures of 220 to 260° C. in a pressure reactor and with removal of the water of reaction formed.
U.S. Pat. No. 2,593,001 describes a continuous process for preparing diaryl sulfones by reacting aromatic sulfonic acids with aromatics, wherein the water of reaction is removed continuously from the reaction zone by the aromatic compound added in gaseous form in countercurrent.
U.S. Pat. No. 2,971,985 discloses the synthesis of dichlorodiphenyl sulfone using SO3, dimethyl sulfate and monochlorobenzene.
The syntheses to prepare dichlorodiphenyl sulfone form not only the desired 4,4′-dichlorodiphenyl sulfone, but different amounts of the 2,4′- and 3,4′-isomers are always obtained, which are referred to hereinafter collectively as incorrect isomers of 4,4′-dichlorodiphenyl sulfone. In order to arrive at a 4,4′-dichlorodiphenyl sulfone usable in polymerizations, it has to be isolated in very pure form (typically >99.0% by weight).
Mixtures of dichlorodiphenyl sulfone isomers can be worked up, for example, by crystallization with/from alcohols, such that increased purities of the desired 4,4′-dichlorodiphenyl sulfone are obtained. EP-A 279387 describes this type of purification by recrystallization.
A further means of removing the incorrect isomers is the chromatographic separation, described in U.S. Pat. No. 4,876,390, of the isomer mixture.
The whereabouts of the undesired incorrect isomers of 4,4′-dichlorodiphenyl sulfone, which are unsuitable for the polymerization, and of the by-produced isomers of monochlorobenzenesulfonic acids, i.e. 2-chlorobenzenesulfonic acid, 3-chlorobenzenesulfonic acid and 4-chlorobenzenesulfonic acid, are not mentioned in any case.
In principle, the undesired isomers can be discarded after discharge from the process. This can be done by discharging a portion of the mother liquor of the crystallization or after removal of the incorrect isomers from the mother liquor. Both reduce the yield of the process based on the feedstocks considerably. Moreover, disposal costs additionally arise. In this context, the aspect of environmental protection (correct disposal of chlorinated aromatics) cannot be neglected.
There is thus a considerable interest in not discarding incorrect isomers from the dichlorodiphenyl sulfone synthesis but in converting them to 4,4′-dichlorodiphenyl sulfone, in order thus to increase the yield.
Isomerization reactions proceeding from aryl sulfones are known per se from the prior art. In Zhurnal Organicheskoi Kimii, 1976, 12 (2), 397, and 1976, 13 (6), 1204, V.A. Kozlov et al. report the isomerization and splitting of tolyl phenyl sulfones and xylyl phenyl sulfones in 92.3% sulfuric acid. However, a conversion of the incorrect isomers from the dichlorodiphenyl sulfone synthesis to 4,4′-dichlorodiphenyl sulfone is not possible in this way.